Fatal Horse Diseases on the Rise in Florida

A virus deadly to horses is spreading rapidly across Florida, infecting more horses this year than in all of 2023.

Nineteen cases of Eastern equine encephalitis (EEE) were recorded in 2024: 10 in June and three in July. There were only 17 cases in 2023. The virus can also infect humans, and the resulting illness can have a mortality rate of up to 1 in 3, according to the Centers for Disease Control and Prevention.

EEE is transmitted primarily through the bite of infected mosquitoes of the Aedes, Coquillettidia, and Culex species. These mosquitoes act as vectors between virus-infected birds and other animals, with the virus settling in their salivary glands and being injected into a human or other animal when the mosquito bites.

The virus is not transmitted directly between horses, from horses to humans, or from birds to humans: the mosquito is needed for the disease to be transmitted, and the virus must come from a bird, as humans and horses are considered “dead-end” hosts.

The Florida Department of Agriculture and Consumer Services’ reportable equine disease map shows three horses have tested positive for the virus in Holmes County, Florida, two have tested positive in each of Alachua, Levy, Madison, Marion and Osceola counties, and one has tested positive in Columbia, Duval, Highlands, Jackson, Polk and Suwannee counties.

In horses, as in humans, the virus can be fatal. In horses, EEE can cause severe neurological symptoms, including fever, circling or pressing on the head, inability to swallow, severe depression, and lack of coordination. According to UC Davis Veterinary Medicine, up to 90% of infected horses die from the disease.

“It’s not common to see very mild cases,” Brittany Welch, an associate veterinarian at Ocala Equine Medical Center, told Florida news station WCJB. “There have been reports of horses surviving, but it’s a pretty preventable disease and typically the cases we see are pretty severely affected.”

Only 4% of people infected with the virus develop symptoms. However, infected people may experience mild flu-like symptoms, including fever, chills, and malaise. Symptoms usually appear a week after infection.

In some cases, EEE can progress to a serious neurological disease, causing encephalitis (inflammation of the brain).

“Overall, only 4 to 5 percent of human infections with Eastern equine encephalitis virus result in Eastern equine encephalitis,” the CDC said.

Symptoms can include high fever, headache, neck stiffness, confusion, seizures and coma. The disease can be fatal or lead to long-term neurological complications.

“Approximately 30 to 45 percent of people with EEEV encephalitis will die from the disease, making it one of the most serious mosquito-borne diseases in the United States,” Florida Health says.

Approximately 11 human cases of EEE are reported each year in the United States, with most cases occurring in the Southeast and along the East Coast. No human cases have been reported as of 2024.

“Transmission of Eastern equine encephalitis virus is most common in and around freshwater hardwood marshes in the Atlantic and Gulf Coast states and the Great Lakes region,” the CDC said.

There is no specific antiviral treatment for EEE, and care focuses primarily on relieving symptoms and managing complications. Although there is no human vaccine for EEE, a vaccine is available for horses that can prevent them from becoming infected.

“The vaccine is very effective at preventing disease, so most of the time when we see cases of Triple E, it’s horses that didn’t get the vaccine or were vaccinated inappropriately or inadequately,” Welch said.

For humans, using insect repellents and wearing protective clothing can help prevent infection.

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